I think it has slightly more to do with where he got hit. I think CC's kick hit him in a spot where it didn't really wobble his head much, considering how freakin thick Hunt's neck/skull is. As where Manhoef hit him dead on the button.

I think it's time. Hunt has taken much punishment right on the jaw with no probs before. Manhoef - stud though he is - is smaller than everyone Hunt has fought before.
Also note he outwrestled Fedor, and slipped out of an Fed-armbar, but later in his career just let Overeem get him with an easy headlock.
Many fighters have lost their ability to take a punch with time. W.Silva springs to mind. I think Chuck has a bit too. CC himself is far more hurtable than he used to be, etc
I don't imagine it's mental - just the brain itself can only take so much before it starts getting more and more easily ko'd

To be honest I don't see how Hunt has taken any sort of considerable damage in his career. He's only had 10 MMA fights and like 20 K-1 fights. And he certainly hasn't taken the sort of beatings someone like Nog or Wandy has. So I wouldn't say he's punch drunk, or has a battered chin.